1. Field of Invention
The present invention relates in general to a nitride deposition process carried out during integrated circuit production. More specifically, the invention pertains to a low-pressure chemical vapor deposition (LPCVD) nitride deposition process.
2. Description of Related Art
Silicon nitride deposition is frequently used in the process of manufacturing an integrated circuit. A conventional LPCVD silicon nitride deposition is shown schematically in FIG. 1 (Prior Art). Each set of twenty-five silicon wafers 10 is disposed on a quartz boat 12. Boat 12 is well known by those skilled in the art and is therefore not shown in detail in FIG. 1. A paddle 14 carries seven boats 12 into a quartz furnace 16. Dichlorosilane (SiCl.sub.2 H.sub.2) gas is sent into the furnace 16 via a gas inlet 18. Ammonia (NH.sub.3) gas is sent into the furnace 16 via a gas inlet 19. An exit 15 of furnace 16 is connected to a vacuum pump. The chemical reaction of silicon nitride deposition is: EQU 3SiCl.sub.2 H.sub.2 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +6HCl+6H.sub.2
Usually, the temperature of furnace 16 is set to about 300.degree. C. when idling. When silicon nitride deposition begins, the temperature is rampaged up to about 780.degree. C.
In the conventional LPCVD process, the silicon wafers disposed on the first and the last boats 12 carried by paddle 14 are dummy wafers 10'. Production wafers 10 are disposed on the other five boats 12 between the first and the last boats. The purpose of using dummy wafers is to increase the uniformity of the thickness of the silicon nitride film on the production wafers which are close to the front end or the rear end.
The dummy wafers can be re-used. However, as known by those skilled in the art, usually the dummy wafers can be re-used only four times, then they must be returned and processed for further use. This is due to the fact that there is also silicon nitride deposited on the dummy wafers. The maximum allowable thickness of the silicon nitride film deposited on the dummy wafers is 0.9 .mu.m because the silicon nitride film may peel off when its thickness is larger than 0.9 .mu.m. If too much silicon nitride film peels off, the furnace and the production wafers will be contaminated seriously, then the furnace must be rampaged down to be cleaned. As a result the down time will increase greatly, and the efficiency of production will be greatly affected.
Furthermore, fifty (50) dummy wafers are used in one LPCVD silicon nitride process. Therefore plenty of dummy wafers must be prepared for use, even when they can be recycled after treatment. As a result, the cost of the management and treatment of the dummy wafers is great.